A network solid or covalent network solid (also called atomic crystalline solids or giant covalent structures)[1][2] is a
chemical compound (or element) in which the atoms are bonded by
covalent bonds in a continuous network extending throughout the material. In a network solid there are no individual
molecules, and the entire
crystal or
amorphous solid may be considered a
macromolecule. Formulas for network solids, like those for
ionic compounds, are simple ratios of the component atoms represented by a
formula unit.[3]
Examples of network solids include
diamond with a continuous network of carbon atoms and
silicon dioxide or
quartz with a continuous three-dimensional network of SiO2 units.
Graphite and the
mica group of
silicate minerals structurally consist of continuous two-dimensional sheets covalently bonded within the layer, with other bond types holding the layers together.[3] Disordered network solids are termed
glasses. These are typically formed on rapid cooling of melts so that little time is left for atomic ordering to occur.[4]
Properties
Hardness: Very hard, due to the strong covalent bonds throughout the lattice (deformation can be easier, however, in directions that do not require the breaking of any covalent bonds, as with flexing or sliding of sheets in graphite or mica).
Melting point: High, since melting means breaking covalent bonds (rather than merely overcoming weaker intermolecular forces).[5]
Solid-phase
electrical conductivity: Variable,[6] depending on the nature of the bonding: network solids in which all electrons are used for
sigma bonds (e.g. diamond, quartz) are poor conductors, as there are no delocalized electrons. However, network solids with delocalized
pi bonds (e.g. graphite) or
dopants can exhibit metal-like conductivity.
Liquid-phase electrical conductivity: Low, as the macromolecule consists of neutral atoms, meaning that melting does not free up any new charge carriers (as it would for an ionic compound).
Solubility: Generally insoluble in any solvent due to the difficulty of solvating such a large molecule.
^Zarzycki, J. Glasses and the vitreous state, Cambridge University Press, New York, 1982.
^Ebbing, Darrell D., and R.A.D. Wentworth. Introductory Chemistry. 2nd ed. Boston: Houghton Mifflin, 1998. Print.
^Brown, Theodore L.; LeMay, H. Eugene Jr.; Bursten, Bruce E.; Murphy, Catherine J. (2009). Chemistry: The Central Science (11th ed.). Upper Saddle River, NJ:
Prentice Hall. pp. 466–7.
ISBN978-0-13-600617-6.